Guess number of classes from logits' shape when nb_classes is missing. (#926)
Guess number of classes from logits' shape when nb_classes is missing. (#926)

* Guess number of classes from logits' shape when nb_classes is missing.

* Guess number of classes from logits' shape when nb_classes is missing.

* PEP8 fix.

* Guess number of classes from logits' shape when nb_classes is missing.

cleverhans-lab/cleverhans,cleverhans-lab/cleverhans,cleverhans-lab/cleverhans,openai/cleverhans